1. Field of the Invention
The present invention relates to a mechanism for controlling a transmission of data in a fragmentation transmission mode where plural data fragments or data frames are transmitted consecutively between a source station and a destination station. Specifically, the present invention is related to an apparatus, a method and a computer program product which provide the necessary means for an improved mechanism for controlling a data transmission in a communication network, such as a wireless local are network, when a fragmented transmission of data is employed.
2. Related Background Art
Prior art which is related to this technical field can e.g. be found in technical specifications related to IEEE 802.11 standard for WLAN networks.
The following meanings for the abbreviations used in this specification apply:    ACK acknowledgement    CCA clear channel assessment    CRC cyclic redundancy check    CTS clear to send    DIFS distributed interframe space    DST destination station    HDR header    MAC medium access control    MPDU MAC protocol data unit    MSDU MAC service data unit    NAV net allocation vector    PIFS point interframe space    PLCP physical layer convergence protocol    PPDU PLCP protocol data unit    RTS ready to send    SIFS short interframe space    SNR signal-to-noise ratio    STA station    SRC source station    WLAN wireless local area network
In the last years, an increasing extension of communication networks, e.g. of wire based communication networks, such as the Integrated Services Digital Network (ISDN), DSL, or wireless communication networks, such as the cdma2000 (code division multiple access) system, cellular 3rd generation (3G) and fourth generation (4G) communication networks like the Universal Mobile Telecommunications System (UMTS), enhanced communication networks based e.g. on LTE or LTE-A, cellular 2nd generation (2G) communication networks like the Global System for Mobile communications (GSM), the General Packet Radio System (GPRS), the Enhanced Data Rates for Global Evolution (EDGE), or other wireless communication system, such as the Wireless Local Area Network (WLAN), Bluetooth or Worldwide Interoperability for Microwave Access (WiMAX), took place all over the world. Various organizations, such as the 3rd Generation Partnership Project (3GPP), Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN), the International Telecommunication Union (ITU), 3rd Generation Partnership Project 2 (3GPP2), Internet Engineering Task Force (IETF), the IEEE (Institute of Electrical and Electronics Engineers), the WiMAX Forum and the like are working on standards for telecommunication network and access environments.
Especially in wireless communication networks, such as for example a WLAN based on IEEE 802.11 standards or comparable network types, in order to obtain a good communication quality, it is required to combat with imperfect wireless channel, e.g. a low signal-to-noise ratio (SNR) or strong neighboring interference, and to reduce overhead of retransmission. One approach used in this connection is a fragmentation transmission scheme. The idea of using fragmentation is to divide a large data packet e.g. in the form of a MSDU into several smaller consecutive packets or frames, such as MAC level frames (MPDUs). The effect of this division or fragmentation is that if the channel quality during the transmission time of one or a few number of the (separated) fragments is not sufficient, this will not destroy the whole MSDU transmission. That is, if retransmission of one or more fragments is required, the source station only needs to transmit those portions of the MSDU instead of the whole MSDU.
For example, a 1500 byte data frame sent using the 1 Mbps 802.11b rate takes 12 ms to transmit, making it susceptible to changing channel conditions. A bit error in the frame would result in the entire frame being retransmitted. When using a fragmentation transmission scheme, the MSDU is broken into smaller sections and each section may be encapsulated in an MPDU. Each MPDU may be sent in a separate PLCP PDU (PPDU) with the preamble of each PPDU providing a new channel estimate. A bit error would result in only the MPDU carrying the erroneous segment being retransmitted.
It should be noticed that using fragmentation mode also introduces the extra overhead due to a necessary security distance between transmissions of frames, such as an additional SIFS time interval, and the frame header of each fragment. However, when the channel condition is poor, the fragmentation mode may still be able to get more benefit from the retransmission reduction than the cost due to those extra overhead. On the other hand, when the channel condition is good, the fragmentation mode may be deactivated.
In the following, channel conditions or settings such that a fragmentation transmission mode is set are assumed.